The information provided below is not admitted to be prior art to the present invention, but is provided solely to assist the understanding of the reader.
Acrylate, methacrylate and other unsaturated monomers are widely used in coatings, adhesives, sealants, and elastomers, and may be crosslinked by ultraviolet light in the presence of photoinitiators or by peroxide-initiated free radical cure. These photoinitiators and/or peroxides are typically low molecular weight multifunctional compounds that may be volatile or readily absorbed through skin and can cause adverse health effects. Functionalized oligomeric photoinitiators may overcome some of these drawbacks; generally, polymeric photoinitiators are nonvolatile compounds, not readily absorbed through skin. However, multistep syntheses may be required, low functionality may be detrimental to reactivity and final properties, and catalyst or initiator may still be required to effect crosslinking.
The Michael addition of acetoacetate donor compounds to multifunctional acrylate receptor compounds to make crosslinked polymers has been described in the literature. For example, Mozner and Rheinberger reported the Michael addition of acetoacetates to triacrylates and tetraacrylates. (16 Macromolecular Rapid Communications 135 (1995)). The products formed were crosslinked gels. In one such reaction, depicted in FIG. 1, Mozner added one mole of trimethylol propane triacrylate (TMPTA) having 3 functional groups to one mole of polyethylene glycol (600 molecular weight) diacetoacetate (PEG600-DAA) having two functional groups. (Each acetoacetate “functional group” reacts twice, thus each mole of diacetoacetate has four reactive equivalents.) The resulting network is considered “gelled”, or cured, despite the presence of unreacted acrylic functional groups. While further reaction can be promoted, this network cannot be made liquid either with heat or solvent because it is essentially crosslinked.
The reaction can be characterized by various ratios to describe the reactants: a mole ratio of TMPTA: PEG 600 DAA=1:1, a functional group ratio of the number of acrylate to acetoacetate functional groups=3:2, and a ratio of reactive equivalents=3:4
U.S. Pat. Nos. 5,945,489 and 6,025,410, to Moy et al. and assigned to the assignee of the present invention, disclose that certain soluble liquid uncrosslinked oligomers, made by one step Michael addition of β-dicarbonyl donor compounds (e.g., acetoacetates) to multifunctional acrylates, can be further crosslinked using ultraviolet light or electron beam radiation without requiring costly photoinitiators. Moreover, when precise proportions of multifunctional acrylate acceptor compounds to β-dicarbonyl donor compounds are combined in the presence of a basic catalyst, liquid oligomeric compositions result. If proportions below the ranges disclosed in the above-cited patent documents are used, crosslinked gels or solid products result. In addition, the disclosed liquid oligomer compositions can readily be applied to various substrates using conventional coating techniques such as roll or spray prior to radiation cure.
Multifunctional acrylates and methacrylates are commonly utilized in the preparation of crosslinked films, adhesives, foundry sand binders, and other composite materials. The invention disclosed herein demonstrates the advantageous use of these uncrosslinked resins alone or modified by reaction/blending with additional materials in coatings applications on a variety of metal substrates. These additional materials include a variety of acrylic monomers and oligomers, primary, secondary and tertiary amines, acid-functional monomers, silicones, elastomers, waxes, and others to modify and improve coatings performance.
Coatings for metal substrates based on the resins described above can be cured by all methods typically used to crosslink acrylic-functional materials. Cure, or crosslinking, is usually accomplished through a free radical chain mechanism, which may require any of a number of free radical-generating species such as peroxides, hydroperoxides, REDOX combinations, and other materials that decompose to form radicals, either when heated, or at ambient temperature in the presence of an amine and a transition metal promoter. Ultraviolet (UV) light or electron beam (EB) radiation are alternative means of initiating reaction by decomposing an appropriate photoinitiator to form free radicals.
The coatings described in this invention offer significant advantages over coatings based on traditional multifunctional acrylic monomers and oligomers in that they can be cured by exposure to UV radiation without the addition of a photoinitiator. Under typical UV curing conditions (˜500 mJ/cm2), these coatings can be effectively cured on a variety of metal substrates with little or no added photoinitiator. Traditional multifunctional acrylates and/or oligomers will not cure upon exposure to such low doses of UV radiation unless a photoinitiator, often at relatively high levels, is added to coating formulations. Traditional photoinitiators (e.g., benzophenone) can be toxic and expensive. An additional disadvantage is that photoinitiators and/or their decomposition products may contribute to film color, which can limit applicability of the coating over white and light-colored substrates.
The novel coatings disclosed here exhibit performance properties that make them very effective across a range of metal substrates. Traditionally, to modify the properties of photoinitiator-containing coating formulations one must admix additives, including reactive monomers or oligomers. Traditional additives can confer higher cost and may compromise some performance attributes. However, the specific properties of the coatings resulting from the present invention can be extensively modified merely by varying oligomer composition rather than by blending with additives. Coating films can be engineered to exhibit wide ranges of hardness, toughness, flexibility, tensile-strength, stain resistance, scratch resistance, impact resistance, solvent resistance, etc. Almost any desired coating performance parameter can be attained by proper selection of the raw material building blocks used to make the oligomer.
Cure of conventional polyacrylate coating systems may be achieved without a UV photoinitiator. However, such systems typically require the use of an expensive, high-energy source, such as electron beam radiation, and cannot be accomplished with much cheaper UV radiation. Full cure can be realized with little or no traditional photoinitiator when the inventive oligomers are formulated into UV-curable coatings.
A coating must adequately wet out the surface of a substrate for it to adhere well to that surface. There are three principle wetting phenomena that apply to coatings: spreading, adhesional, and penetrational or immersional wetting. Spreading and adhesional wetting directly impact the application of a coating to a particular surface. Penetrational or immersional wetting impacts the application of coatings to porous surface structures and to particulate dispersions. When a coating fluid wets a surface, a second fluid, usually air, is displaced. Surface tension, both of the coating fluid and of the substrate, controls the action of wetting.
The spreading of a liquid over a solid is defined by SL/S=γSA−(γLA+γSL), where, γSA denotes the surface tension of the substrate under air, γLA denotes the surface tension of the liquid coating under air, and γSL denotes the interfacial tension or free energy of the substrate/liquid coating interface. A coating fluid will spread spontaneously when SL/S is either positive or zero. Where SL/S is negative, the coating will not properly wet the substrate. The resultant coating will be characterized by pinholes, fisheyes, or picture framing, and in the worst case scenario, complete de-wetting (‘beading’) will occur. The substrate-air surface tension cannot be controlled by the resin designer and the substrate-coating interfacial tension is assumed to be a minimum when the surface tensions of the substrate and coating fluid are nearly identical. Therefore, for best wetting, the coating surface tension should be lower than, but approximate equal to the surface energy of the substrate. Preferably, the surface tension of a coating resin should be about 3 to 10 dynes/cm less than the surface energy of the substrate.
The term adhesion refers to the attraction that molecules of one material experience towards molecules of a different material. The attraction of molecules of one material towards other molecules of the same material is cohesion. The surface tension of a liquid is a measure of its cohesion. The analogous term for a solid is surface energy. Surface tension and surface energy have the same units (dynes/cm) and surface tension is often used interchangeably to refer to the liquid or solid state.
The Lewis acid/base theory is the current state of the art in understanding adhesive phenomena. Atoms are held in larger structures called molecules by two types of bonds: ionic and covalent. Similarly molecules are held in larger structures (liquids and solids) by cohesive and adhesive forces termed intermolecular forces. Approximately twenty such forces are known, most are insignificant and may be ignored to a first approximation. The dominant forces are primarily electrostatic. The theory divides intermolecular forces into two principal groups. The various names have fine shades of meaning, but are normally used interchangeably: a) LW=Liftshitz-van der Waals≈London≈non-polar≈dispersive forces; and b) AB=(Lewis) acid/base≈polar forces. Dispersion forces are always present, but acid/base forces, which may or may not be present, contribute most to industrial adhesion.
FIG. 3 is a schematic drawing showing that clean metal surfaces are substantially covered with free OH-groups. These groups confer the potential for adhesion based on Lewis acid-base electrostatic interactions.
A need therefore exists for UV-curable metal coating resins that have surface tensions and electrostatic character appropriately compatible with the surface energy of metals in order to allow effective wetting and adhesion on those metal surfaces.
Other objects and advantages will become apparent from the following disclosure.